(a) Field of the Invention
The present invention relates to an optical controller. More particularly, the present invention relates to a device and method for automatically controlling optical power provided by a light source, and an optical extinction ratio (ER).
(b) Description of the Related Art
In a general optical communication network, it is required to maintain outputs and an extinction ratio (an intensity ratio of the minimum transmission light and the maximum transmission light) of a laser diode and an optical transmitter that are determined according to data rates and transmission distances in order to provide quality optical signal transmission. The laser diode generally used as a light source in the optical communication starts being oscillated when a current of more than a threshold current is applied at a predetermined temperature, and the optical output of the laser diode is linearly increased as the applied current value is increased. A bias current is applied at the oscillation in order to maintain the average optical output of the laser diode, and a modulation current is applied in order to maintain a predetermined extinction ratio.
The optical output/current characteristic curve of the laser diode is variable depending on the temperature and the aging of the laser diode. FIG. 1 shows a graph for an optical output/current characteristic curve for a general laser diode. In FIG. 1, Ib is a bias current value, and Im is a modulation current value. P0 is an optical output value of the laser diode when the bias current is applied only, and P1 is an optical output value when the modulation current is applied. Also, Pav is an average optical output value, which is an average of P0 and P1. In this instance, the extinction ratio is defined by Equation 1.ER=10 log(P1/P0)[dB]  (Equation 1)
Referring to FIG. 1, the optical output/current characteristic curve for the laser diode varies according to the temperature. In detail, when a predetermined modulation current is applied to the laser diode so as to acquire a desired extinction ratio with reference to the optical output/current characteristic curve at room temperature (e.g., 25° C.), the slope of the optical output/current characteristic curve varies even under the same conditions when the laser diode has a high temperature (e.g., 85° C.). As a result, the high temperature has less optical output value, thus reducing the extinction ratio. The reduction of the extinction ratio influences the power penalty in the optical communication to reduce the maximum transmission distance.
Therefore, it is required to maintain the extinction ratio so as to provide optimized optical communication, and it is needed to control the amplitude of the modulation current and that of the bias current according to the optical output/current characteristic curve.
In general, in order to maintain the extinction ratio, the modulation amplitude is controlled as follows.
The first method is to find characteristic values of the optical output/current characteristic curve depending on the temperature changes of the laser diode, storing an optimized modulation amplitude into a lookup table based on the characteristic values, and controlling the modulation amplitude based on the lookup table. In this instance, when the temperature of the laser diode is changed, corresponding modulation amplitude is read from the lookup table and is then fed back to the laser diode.
However, in this case, the modulation amplitude is determined according to the temperature change from the characteristic value for the optical output/current characteristic curve of the laser diode, and hence, there is a restriction on establishing in detail the difference of characteristic changes caused by the optical output/current characteristic curve and temperature change that are slightly different for respective laser diodes.
Also, there are many restrictions to changing the lookup table, the data of which are statistically processed so as to acquire the desired extinction ratio when considering the costs caused by measurement and control, and time. Therefore, the problem is that the lookup table is only applied to the laser diodes having characteristic values of a similar optical output/current characteristic curve. Further, it is needed to individually change driving circuits of the laser diodes in order to change the optical output/current characteristic curves according to the temperature for the respective laser diodes.
The second method is to apply a pilot tone signal having a predetermined measurement frequency to the original signal output by the laser diode to sense an error signal, and controlling the feedback of the modulation current of the laser diode based on the sensed error signal.
However in the above-noted second method, a circuit for applying the pilot tone signal is added so that the whole circuit structure becomes more complicated and jitter caused by applying the pilot tone signal is increased. Also, since the optical output/current characteristic curve is non-linearly changed at high temperature (e.g., 85° C.), the amplitude of the applied pilot tone signal is reduced. Therefore, the modulation current is increased so as to correct the reduction thereof, and as a result, the temperature of the laser diode is further increased so that normal operation is not performed.
As described above, the conventional manual extinction ratio controlling method and the pilot tone based extinction ratio controlling method fail to control the current by applying different optical output/current characteristics according to the temperature changes of the respective laser diodes. Therefore, when the laser diode is used at a high temperature or a low temperature, errors occur to deteriorate communication quality of the communication system and generate substantial communication problems.